1. Field of the Invention
This invention relates to a system and method for constructing composite building boards. More particularly, the present invention relates to the use of thermoplastic coatings in the construction of composite gypsum building boards.
2. Description of the Background Art
Building board, also known as wallboard, plasterboard, or drywall, is one of the most commonly used building components in the world today. Building board is frequently used within the interior of a dwelling, where it functions both as a finished wall covering and as a structural room partition. Building board can also be used on the exterior of a dwelling, where it serves as a sheathing to provide weather protection and insulation. Building board can also be used as an interior facing for other structures as well, such as stairwells, elevator shafts, and interior ducting.
One particularly popular form of building board is known as glass reinforced gypsum (GRG) board. An example of one such board is disclosed in U.S. Pat. No. 4,265,979 to Baehr et. al. Baehr discloses a building board constructed from opposing glass fiber mats with an intermediate gypsum core. This construction provides a hardened external surface and is an improvement over earlier paper faced boards.
Current GRG manufacturing techniques have some significant drawbacks. Namely, during construction, some of the individual mat fibers are not covered by the gypsum slurry core and are therefore exposed. These fibers have a tendency to dry out and disengage from the board. As a result, free floating glass fibers tend to accumulate on and damage associated forming equipment, such as forming tables, forming plates, motor drives, bearings, and the like. The presence of disengaged fibers also presents a significant hazard to workers who must wear appropriate safety masks so as not to ingest the fibers. The most common way to combat this problem is through the use of expensive dust collection equipment and/or the periodic and repeated cleaning of the forming equipment.
A subsequent board manufacturing technique is described in commonly owned U.S. Pat. No. 4,378,405 to Pilgrim. The contents of the Pilgrim patent are fully incorporated herein by reference. Pilgrim discloses a GRG board that is faced on one or both sides with a porous, nonwoven glass mat. The glass mat of Pilgrim is slightly but fully embedded into the slurry core. This is accomplished by vibrating the gypsum slurry to cause it to pass through the porous openings in the mat. Embedding the mat within the core as taught in Pilgrim results in a thin film of slurry being formed on the outer surface of the board. Building boards with this construction are referred to as embedded glass reinforced gypsum (EGRG) boards.
EGRG boards eliminate, or greatly reduce, the presence of exposed fibers and greatly reduce the presence of free floating fibers. However, the construction of EGRG boards also has its drawbacks. Namely, EGRG boards require the application of low viscosity gypsum slurry. This slurry leaks from the boards during manufacture and accumulates on associated forming equipment. Thus, during manufacture, the forming tables, forming belts, and associated rollers and motors are exposed to substantial build-ups of gypsum slurry. Over time, if not regularly cleaned, the manufacturing process comes to a complete stop. Thus, in traditional GRG and EGRG building board manufacturing techniques there is a substantial capital investment in equipment designed to clean the forming areas.
Additionally, even in the construction of EGRG boards, there is a continuing problem with some fibers becoming exposed, dried and detached. This, in turn, results in the accumulation of free fibers on the forming tables, forming belts and associated rollers and motors. As with the excess gypsum slurry, these fibers must be removed in order to prevent equipment failure resulting in downtime.
Thus, there exists a need in the art for improved building board manufacturing techniques. More specifically, there is a need in the art for manufacturing techniques that minimize the accumulation of gypsum slurry and/or free floating fibers on associated forming equipment. There also exists a need to minimize capital investment needed to construct GRG and EGRG building boards. There is yet another need to economically produce GRG and EGRG building boards with improved physical characteristics. The present invention is aimed at achieving these objectives.